The present invention relates to an electrode for forming minute holes in a workpiece through an electrical discharge machining (EDM) or electroerosive machining process, and an EDM apparatus employing such an electrode.
There are two EDM machining processes for forming a minute hole having a diameter of 0.1 mm or smaller in a workpiece. In one process, the machining electrode is held at rest, and in the other process, the machining electrode is rotated about its own axis. Rotating the machining electrode could increase the machining speed, and improve out-of-roundness and accuracy of the shape of the hole formed. However, the machining process in which the electrode is held at rest during operation has heretofore been prevalent in the art since the electrode is not subjected to displacement and the time required for the electrode to form the hole is shorter.
Various electrode constructions are known for use in forming holes while the electrode is held at rest. Where a thin wire which requires no cutting is employed as an electrode, it is directly soldered to an electrode holder, or is plated, reinforced, and chucked, or protected and guided by a body of glass, ruby or the like and chucked. Where a relatively thick wire is used for an electrode, the tip end thereof is thinned by electrolytic grinding so as to serve as a machining electrode.
The above electrode constructions however fail to provide a degree of concentricity of a few microns or less between the machining tip and outer periphery of the electrode. It has been impossible to use such electrode in machining a hole while rotating the electrode since it is difficult to attach the electrode perpendicularly to the workpiece.
When a workpiece is subjected to an EDM process with the wire electrode held at rest, the shape of the wire electrode is transferred to the workpiece. No hole with small out-of-roundness would be formed in the workpiece if a commercially available thin wire finished to insufficient dimensional accuracies were used for machining.
One electrode construction used for machining a workpiece while the electrode is rotating is composed of a round rod having a diameter of about 1 mm which can be chucked. The electrode material as it is rotated is shaped into an electrode of a desired diameter by a reverse electrical discharge. Although the electrode thus formed has sufficient accuracy, it will take about one hour to form an electrode for forming a minute hole 30 micrometers across from an electrode material having a diameter of 1 mm. The thinned portion of the electrode which has a diameter of 30 micrometers is short in length and hence in service life. Because the electrode will be displaced once detached from the chuck, the electrode has to be formed again each time it machines a minute hole in a workpiece.